Method for producing a leather semi-finished product

ABSTRACT

A semifinished leather product is produced from an animal hide by pretanning with the use of a chromium-free tanning agent and of a clay mineral which, after stirring for 30 minutes in water at 50° C. at a circumferential rotor speed of from 5 to 25 m/s, has a number average particle diameter of less than 2 μm or a bimodal particle size distribution with a first, finely divided fraction whose number average particle diameter is less than 0.5 μm and a second, coarser fraction whose number average particle diameter is less than 5 μm, determined in each case by the method according to ISO 13320-1, by combined laser light diffraction and light scattering, a proportion of the first, finely divided fraction being from 10 to 90% by weight, and drying of the pretanned animal hide to a water content of from 5 to 45%, based on the total weight of the semifinished leather product.

The present invention relates to a process for the production of asemifinished leather product.

In the leather industry, the readily perishable raw material, the animalhide, must be converted into a biologically and mechanically stabilizedstate in order to be storable and transportable. For this purpose,semifinished leather products are produced by pretanning, as a rule bythe following two, basically different processes:

On the one hand, by pretanning with corrosion-containing tanning agentsto give the wet blue semifinished product and, on the other hand, bychromium-free pretanning to give the wet white semifinished product.

Biocides can be added to semifinished leather products in the moiststate and then, if a maximum temperature of 25° C. is not exceededduring transport and storage, said semifinished products can be storedfor up to about 1 year.

Wet blue semifinished products have the disadvantage that they arealready chromium-containing, i.e. that further processing to give achromium-free leather is not possible and the wastes from the furtherprocessing, for example shaving, have to be worked up and disposed of byan expensive procedure. Moreover, the water content cannot be reducedbelow about 40 to 80% by weight since, on drying to lower watercontents, the appearance of the skin is irreversibly changed: creasesare fixed and subsequent dyeing attempts lead to a nonlevel appearance,i.e. the quality is irreversibly reduced by attempts to dry to lowerwater contents. Resoftening without a reduction in quality is notpossible.

In contrast, wet white semifinished products have the advantage thatthey are not contaminated with the heavy metal chromium, but they arevery susceptible to attacks by microorganisms, in particular by molds,and are therefore considered to be virtually untransportable.

In addition, wet white semifinished products practically cannot be driedto a water content below about 40 to 80% by weight since they dry to ahorny material with fiber adhesion, and resoftening is thus impossible.

Known semifinished leather products have to be biologically stabilizedwith addition of biocides. However, the use of biocides, which have beennecessary to date for stabilizing semifinished leather products, is,apart from their price, not unproblematic since some biocides may alsobe toxic, sensitizing or allergenic to humans and/or eliminatetoxicologically harmful substances, in particular formaldehyde.

The main problem associated with known semifinished leather products istherefore that the water content must not fall below a certain level ifthe quality of the leather to be produced in the subsequent processsteps is not to be adversely affected. On the other hand, however, thisstill relatively high water content leads to correspondingly hightransport costs and in particular to limited transportability andstorability since such semifinished leather products are ideal culturemedia for microorganisms.

There has therefore been no lack of attempts to provide processes forthe production of semifinished products, which processes meet thesecontradictory requirements, i.e. reduce the water content to a range inwhich there is no longer any threat of attack by microorganisms and atthe same time the quality of semifinished leather products is notadversely affected.

EP-B 0 281 486 describes a process for the biological stabilization ofpickled pelts, in which a residual water content of only about 20% byweight can be achieved by replacing water by a substance selected fromthe group consisting of the polyalcohols, the condensates of fattyalcohols with ethylene oxide and the condensates of alkylphenols withethylene oxide. However, this process has the substantial disadvantagethat substances which have surface-active properties and determine thebasic character of the semifinished product and adversely affectsubsequent tanning steps, for example by influencing the fatdistribution, are introduced for stabilizing the semifinished product.Moreover, such semifinished products have a relatively low shrinkagetemperature in the region of about 50° C.

It is an object of the present invention to provide a process for theproduction of a biologically stable semifinished leather product, whichprocess is environmentally friendly, does not adversely affect thequality of the semifinished product and end products produced therefromand does not impose any restrictions at all in the further processing.

We have found that this object is achieved by a process for theproduction of a semifinished leather product from an animal hide bypretanning with a chromium-free tanning agent, wherein pretanning iseffected with the additional use of a clay mineral which, after stirringfor 30 minutes in water at 50° C. at a circumferential rotor speed offrom 5 to 25 m/s, has a number average particle diameter of less than 2μm or a bimodal size distribution with a first, finely divided fractionwhose number average particle diameter is less than 0.5 μm and a second,coarser fraction whose number average particle diameter is less than 5μm, in each case determined by the method according to ISO 13320-1, bycombined laser light diffraction and light scattering, the proportion ofthe first, finely divided fraction being from 10 to 90% by weight, andwherein the pretanned animal hide is dried to a water content of from 5to 45%, based on the total weight of the semifinished leather product.

It has surprisingly been found that semifinished leather products drywith a loose structure and isolated fibers as a result of pretanningwith the use of finely divided clay minerals having the particlediameter as defined above and, after drying, can be readily resoftenedwith pure water without additives.

Clay minerals which, after stirring for 30 minutes in water at 50° C. ata circumferential rotor speed of from 5 to 25 m/s, have a number averageparticle diameter of less than 2 μm or a bimodal size distribution witha first, finely divided fraction whose number average particle diameteris less than 0.5 μm and a second, coarser fraction whose number averageparticle diameter is less than 5 μm, the proportion of the first, finelydivided fraction being from 10 to 90% by weight, are disclosed astanning agents or for the preparation of tanning agents in thenon-prior-published German Patent Application DE 102 37 259.

Clay minerals having the particle sizes defined above form stableformulations for use in chromium-free or chrome tanning which ensureimproved properties of the leather obtained thereby.

A treatment with a sufficient amount, as a rule about 950 ml, of water,based on 50 g of the clay mineral at 50° C. and with vigorous stirring,at a circumferential rotor speed of from 5 to 25, preferably from 10 to20, m/s, for 30 minutes is required for the delamination for determiningthe number average particle diameter, the clay mineral being dispersedin water. For example, a rotor-stator Ultraturrax® mixer from IKA can beused for this purpose. By means of this treatment, it is intended toensure that the clay mineral is delaminated until the particle size nolonger changes.

The determination of the particle sizes and particle size distributionwas effected according to ISO 13320-1 by combined laser lightdiffraction and light scattering using a Malvern 2000 analysisinstrument from Malvern.

It is assumed that, for the action of the clay minerals having theparticle sizes defined above, the interaction thereof with the collagenchains of the hide is essential. This interaction is possible, forexample, via hydrogen bridges between collagen and the surface hydroxylgroups of the clay minerals. It has surprisingly been found that clayminerals having the defined particle sizes are irreversibly incorporatedinto the hide.

Clay minerals are weathering products of primary aluminosilicates, i.e.of compounds having different proportions of aluminum oxide and silicondioxide. Silicon is surrounded tetrahedrally by four oxygen atoms, whilealuminum is present in octahedral coordination. Clay mineralspredominantly belong to the phyllosilicates, also referred to as sheetsilicates or leaf silicates, but in some cases also to the bandsilicates (cf. Römpp Chemie Lexikon, 9th edition, 1995, pages 4651 and4652).

For the novel formulations, it is particularly advantageous if the claymineral is a phyllosilicate. Preferably, the phyllosilicate may be akaolinite, muscovite, montmorillonite, smectite, saponite, vermiculite,hallosite or bentonite, in particular a hectorite or an organicallymodified variant thereof.

Particularly good results are obtained if substances which, owing totheir chemical structure, are capable of forming strong hydrogen bondswith the clay mineral, in particular urea or urea derivatives, alcohols,polyols, propylene carbonate, organic amides, urethanes, saccharides orderivatives of saccharides, in particular nitrocellulose, sulfitecellulose or ethylhexylcellulose, are added to the clay minerals beforeor during the use thereof as tanning agents. Inter alia, thedelamination of the clay mineral, in particular of the sheet silicate,is supported by this treatment.

In addition to the clay mineral, a chromium-free tanning agent,preferably an aldehyde tanning agent, an isocyanate, an aluminum salt,an oxazolidine or tetrakishydroxymethylphosphonium chloride, is used forthe pretanning.

Preferably used clay minerals are those which have a number averageparticle diameter of less than 1 μm.

After the novel process, the animal hides pretanned using a finelydivided clay mineral are dried to a water content of from 5 to 45,preferably from 15 to 35, % by weight, based on the total weight of thesemifinished leather product.

By bringing the water content to below 45, preferably below 35, % byweight, the pretanning animal hide is brought to a state which isbiologically substantially stable and can nevertheless be readilyresoftened without comparing the quality of the end product producedtherefrom.

The drying step is in principle not limited with regard to theprocessing conditions: it is possible to effect drying at ambienttemperature and pressure, under reduced pressure and/or at elevatedtemperatures, preferably by drying on a tenter frame. By means of dryingon a tenter frame, substantially flat and hence easily stackablesemifinished products are obtained.

The novel process preferably comprises the further processing step wherethe dried semifinished leather product is resoftened by treatment withwater or an aqueous solution of a tanning assistant, in particular anamphoteric or cationic polymer, preferably by spraying, to a watercontent of from 50 to 80% by weight, based on the total weight of thesemifinished leather product.

A further advantage of the novel process is due to the absorptivity ofthe substantially dried semifinished leather product: thus, aqueoussolutions of tanning assistants having poor affinity to the hide arephysically absorbed and uniformly transported and distributed in theinner hide regions, which has to date lead to unsatisfactory resultsowing to the comparatively low chemical potential when working in anaqueous liquor. The semifinished leather product produced by the novelprocess can therefore be moistened in a further processing step with anaqueous solution of a tanning assistant which is absorbed by means ofphysical forces, in particular osmosis, into the semifinished leatherproduct, preferably generally aqueous solutions of proteins, inparticular protein hydrolysis products, or polymers. This canadvantageously be effected simultaneously with the step of resofteningby spraying or immersion.

According to the invention, a semifinished leather product which iscompletely open with respect to further processing to end product isthus provided. Up to the stage of the semifinished leather product, itis possible to operate in the absence of chromium, the furtherprocessing being possible either in the absence of chromium or by chrometanning. The semifinished leather product can be shaved either prior todrying or after resoftening, chromium-free shaving always beingobtained. The shaving waste, which can amount to up to 30% of thematerial fed to the shaving stage, is unproblematic because it is freeof chromium and therefore need not be disposed of as special waste butcan on the contrary be utilized as fertilizer.

A further advantage is that hide defects or quality-impairingabnormalities of the hide are more readily observable on the driedsemifinished products in terms of defect detection and hence the sortingwith respect to hide quality can be carried out more easily, morequickly and with improved reproducibility. A further advantage is thereduced water content, which leads to obvious economic advantages withregard to transport and storage.

The examples which follow illustrate the invention.

COMPARATIVE EXAMPLE C0 AND EXAMPLES E1 TO E5

The commercial grades shown below under their trade name are productsfrom BASF AG, Ludwigshafen.

COMPARATIVE EXAMPLE C0

Butts of pickled cattle sides, split to a thickness of 3 mm were treatedat a pH of 2.8 to 3.0 at room temperature in succession with 3% ofRelugan® GT 24, a glutaraldehyde solution, for 60 minutes and then with3% of a 50% strength liquid sulfone tanning agent. After a drumming timeof 90 minutes, the basicity was increased to pH 3.8 to 4.0 with a 2:1mixture of Tamol® NA and sodium formate.

EXAMPLES E1 TO E5

In contrast to the pretanning described under C0, in each case 2% byweight, based on the pickle weight, of the clay minerals mentioned belowwere added to the liquors together with the glutaraldehyde solution orimmediately after the metering.

The morphological characteristics are shown in table 1 below. The meanparticle size of the clay mineral was determined by laserdiffraction/light scattering on freshly prepared suspensions each havinga solids content of 5%, according to ISO 13320-1.

Liquor exhaustion and shavability were assessed visually according to arating scale of from 1 to 5. The shrinkage temperature was determinedaccording to DIN 53336, which was modified in the following points, asmentioned below:

-   point 4.1: the samples had the dimensions 3 cm·1 cm, and the    thickness was not determined;-   point 4.2: only one instead of 2 samples was tested per leather    sample;-   point 6: omitted;-   point 7: the drying in the desiccator under reduced pressure was    omitted and-   point 8: the shrinkage temperature was read when the pointer moved    back.

TABLE 1 Mean Liquor Shava- Shrinkage particle exhaustion bility temper-Silicate/ size [Rating [Rating ature Type [μm] 1-5] 1-5] [° C.]Comparative — — 3 3 76 example C0 Example E1 Kaolin 2.3 3.5 3 77 ExampleE2 Mont- 1.3 2 2.5 78 morillonite Example E3 Bentonite 1.1 2 2.0 81Example E4 Muscovite 1.8 2.5 1.5 79 Example E5 Mont- 0.6 1.5 1 82morillonite

The table shows predominantly improved values in the liquor exhaustion,shavability and shrinkage temperature for examples E1 to E5, relative tocomparative example C0.

The semifinished products obtained according to C0 and E1 to E5 weredried and then retanned. For this purpose, the hides obtained accordingto comparative example C0 and examples E1 to E5 were drummed, shaved to1.5 to 1.7 mm and each cut into four strips of equal size, each weighingabout 1 kg.

In the comparative experiments below denoted by the letter a, one stripeach was left in the moist state, i.e. placed on a rack overnight andthen further processed with the other strips of the correspondingexperimental series.

In the experiments denoted by the letter b, one strip each was dried inthe air at 25° C.; in the experiments denoted by the letter c, one stripeach was fixed or clamped on a frame by means of clamps and dried at 45°C. in an air stream. Finally, in the examples denoted by the letter d,one strip each was dried in a dryer for 10 minutes at 45° C. and under areduced pressure of 80 mbar. The residual water was determined using anAqua-Boy® LMIII moisture meter from Schröder Prüfmaschinen KG, Weinheim.

For assessing the quality of the dried semifinished products, the latterwere subjected to retanning to give furniture leather.

The pretanned and dried hide strips were drummed for 3 minutes togetherwith 300%, based on the dry weight, of water. The resoftening behaviorwas rated using a scale from 1 to 5, a good rating indicating that asupple, absorptive hide was obtained without further mechanicaltreatment. Rating 5 shows irreversible hornification of the tissue.

The appearance of the fibers on the flesh side was rated using a scalefrom 1 to 5. Here too, rating 5 represents inhomogeneous fibermorphology with a high proportion of fiber adhesions.

For the retanning, 20% of Relugan® SI were metered and drumming effectedat 30° C. for 10 minutes, followed by 20% of Densodrin® PS, which wasallowed to act for a further 90 minutes. Furthermore, 8% of Basyntan® SWand 4% of Luganil® Brown NGB were used. This was followed byacidification to pH 3.6 with 3% of formic acid for 30 minutes. About 50%less chemicals in the form of tanning assistants were added to the teststrips of the a series.

The leathers were washed twice with 500% of water, sammed, dried in theair and staked. The evaluation of the leather with respect to thequality properties of body, levelness and grain tightness was effectedusing a rating scale from 1 to 5. The tensile strength was determinedaccording to DIN 53328.

TABLE 2 Fiber appear- Grain Water ance Resoft- Level- tight- con- (fleshening Body ness Tensile ness tent side [Rating [Rating [Rating strength[Rating [%] dry) 1-5] 1-5] 1-5] [N] 1-5] C0a 56 — — 3.5 3 226 3.5 C0b 275 5 — — — — C0c 25 5 5 — — — — C0d 28 5 5 — — — — C1a 54 — — 3 2 240 3E1b 24 2.5 2 2.5 2 232 2.5 E1c 25 2 3 2.5 1.5 230 2 E1d 21 2.5 3.5 3 2239 3 C2a 58 — — 2.5 2.5 256 2 E2b 23 2 2 1.5 2 251 2 E2c 23 2 1.5 1.51.5 230 2 E2d 22 3 2 2 2.5 248 2.5 C3a 60 — — 2 2 243 2.5 E3b 25 2 2 2 2234 2 E3c 24 1.5 2 1.5 1.5 228 2 E3d 21 2.5 2.5 2 2 230 3 C4a 57 — — 1.52 274 1.5 E4b 26 1 2 1.5 1.5 265 2 E4c 23 1 1 2 1 260 1 E4d 23 2 1.5 21.5 268 1.5 C5a 56 — — 1 2 269 2.5 E5b 22 1.5 1 1.5 1.5 274 2 E5c 24 2 11 1 267 1.5 E5d 21 2.5 1.5 2 1 276 1.5

The results in table 2 show that, in comparative experiments C0b to C0d,the drying led to an extreme quality deterioration in the fiberappearance on the flesh side and to fiber adhesion during resoftening.No leather end products could be obtained from the dried experimentalstrips of series C0.

A comparison of the a series (without drying) with the driedexperimental strips of series b to d predominantly showed an improvementin the quality features of body, levelness, grain tightness and tensilestrength of the retanning end products.

EXAMPLES 6 TO 9 Retanning with Chromium-Containing Tanning Agents toGive Shoe Leather

Four pickled cattle sides having a split thickness of 3.0 mm wereconverted into the corresponding semifinished product by the wet whitestandard process described above under comparative example C0.

The tanning liquor contains no clay mineral in comparative example C6,whereas in each case 2.5% of montmorillonite powder, corresponding toexample E5, were used in examples 7 to 9.

The pretanned hides were shaved to 1.6 to 1.8 mm after samming. Thehides from examples E8 and E9 were dried overnight on a tenter frame at45° C. and the hides from comparative example C6 and example E7 were, onthe other hand, protected from drying out until further processing.

The resoftening was effected in example E8 by immersion in water (150%)in an appropriate trough. In example E9, the hide was soaked by sprayingwith a 20% strength solution of protein hydrolysis product obtained fromleather wastes and was then placed on a rack for 2 hours.

The further processing of the hides was then effected in a separate drumin each case, together with 150%, based on the shaved weight, of water,about 0.3% of formic acid, to a pH of from 3.2 to 3.5, and 5% ofChromitan® B, the leathers being drummed at 30° C. for 90 minutes.Thereafter, the basicity was raised to pH 3.9 to 4.1 with sodiumbicarbonate, 1% of the dye Luganil® Brown was metered in after theliquor had been changed (100%) at from 25 to 35° C. and drumming waseffected for 10 minutes.

Thereafter, 3% of the polymer tanning agent Relugan® RV, 5% of thesulfone tanning agent Basyntan® DLX and 2% of the resin tanning agentRelugan® DLF were added and drumming was carried out again for 20minutes at 10 revolutions per minute. Furthermore, the hide was treatedfor 40 minutes with 3% of the vegetable tanning agent Mimosa®, followedby treatment with a further 2% of the dye Mimosa® for 40 minutes.

The mixture was acidified to pH 3.6 to 3.8 with formic acid. After 20minutes, the liquor was discharged and washing was effected with 200% ofwater. Finally, 5% of Lipodermlicker® CMG and 2% of Lipodermlicker® PNwere metered in 100% of water at 50° C. After a drumming time of 45minutes, acidification was effected with 1% of formic acid.

The washed leathers were sammed, dried and staked and assessed accordingto the quality features shown in table 3 below.

The chromium exhaustion (in %) is based on the respective availableamount of chromium. The chromium concentration of the liquor wasdetermined titrimetrically. The liquor exhaustion is based on substanceswhich are used in the further retanning steps, in particular resintanning agents, fatliquoring agents or dyes, in each case samples beingtaken after the dyeing and fatliquoring and being visually assessed.

TABLE 3 Stitch tear Cr exhaustion Liquor Grain Tensile strength*resistance** Grain [%] exhaustion Body tightness Softness [N] [N]Levelness appearance C6 82 3 3 2.5 2.5 298 231 3 3 E7 89 2 1.5 1.5 2 323248 2.5 2 E8 91 1.5 2 2 1 309 256 1.5 2 E9 93 1.5 1.5 2 1.5 318 253 1.51.5

The table shows that, in examples E7 to E9, all leather properties areimproved compared with comparative example C6 and moreover the liquorexhaustion is likewise improved.

1. A process for the production of a semifinished leather product froman animal hide, the process comprising: pretanning an animal hide with achromium-free tanning agent to produce a pretanned animal hide, whereinpretanning is effected with the additional use of a clay mineral which,after stirring for 30 minutes in water at 50° C. at a circumferentialrotor speed of from 5 to 25 m/s, has a bimodal size distribution with afirst, finely divided fraction whose number average particle diameter isless than 0.5 μm and a second, coarser fraction whose number averageparticle diameter is less than 5 μm, in each case determined by themethod according to ISO 13320-1, by combined laser light diffraction andlight scattering, the proportion of the first, finely divided fractionbeing from 10 to 90% by weight, and the pretanned animal hide is driedto a water content of from 5 to 45%, based on the total weight of thesemifinished leather product.
 2. The process as claimed in claim 1,wherein the chromium-free tanning agent for the pretanning is selectedfrom the group consisting of an aldehyde tanning agent, an isocyanate,an aluminum salt, an oxazolidine and tetrakishydroxymethylphosphoniumchloride.
 3. The process as claimed in claim 1, wherein the pretannedanimal hide is dried to a water content of from 15 to 35%, based on thetotal weight of the semifinished leather product.
 4. The process asclaimed in claim 1, wherein substances which, owing to their chemicalstructure, form strong hydrogen bonds with the clay mineral, are addedto the clay mineral before or during the use thereof in the pretanning.5. The process as claimed in claim 1, wherein the clay mineral is aphyllosilicate.
 6. The process as claimed in claim 5, wherein thephyllosilicate is selected from the group consisting of a kaolinite, amuscovite, a montmorillonite, a smectite, a saponite, a vermiculite, ahallosite, a bentonite and organically modified variants of thesephyllocilicates.
 7. The process as claimed in claim 1, wherein thedrying is carried out under conditions selected from the groupconsisting of at ambient temperature and ambient pressure, under reducedpressure, at elevated temperatures, and at reduced pressure at elevatedtemperatures.
 8. The process as claimed in claim 1, further comprising:moistening the semifinished leather product with an aqueous solution ofa tanning assistant which is absorbed into the semifinished leatherproduct by means of a physical force.
 9. The process as claimed in claim1, further comprising: resoftening the dried semifinished leatherproduct by treatment with water or with an aqueous solution orsuspension of a tanning assistant, to a water content of from 50 to 80%,based on the total weight of the semifinished leather product.
 10. Theprocess as claimed in claim 2, wherein the chromium-free tanning agent,which is an aldehyde tanning agent, is glutaraldehyde.
 11. The processas claimed in claim 4, wherein the substances are selected from thegroup consisting of urea, an alcohol, a polyol, a propylene carbonate,an organic amide, a urethane and a saccharide.
 12. The process asclaimed in claim 11, comprising the saccharide, wherein the saccharideis selected from the group consisting of nitrocellulose, sulfidecellulose and ethylcellulose.
 13. The process as claimed in claim 6,wherein the bentonite is a hectorite.
 14. The process as claimed inclaim 8, wherein the drying is carried out on a tenter frame.
 15. Theprocess as claimed in claim 8, wherein the physical force comprisesosmosis.
 16. The process as claimed in claim 8, wherein the semifinishedleather product is moistened with an aqueous solution of a proteinhydrolysis product.
 17. The process as claimed in claim 9, wherein thetanning assistant is an amphoteric or cationic polymer.
 18. The processas claimed in claim 9, wherein the resoftening comprises spraying thedried semifinished leather product with the treatment.
 19. The processof claim 17, wherein the tanning assistant is a cationic polymer. 20.The process of claim 17, wherein the tanning assistant is an amphotericpolymer.